Heat hardenable chromium-nickel-aluminum steel



HEAT HARDENABLE CHROMIUMNICKEL.ALUMINUM STEEL Filed Dec. 50, 1960 Oct. 6, 1964 D c. PERRY ETAL 3-Sheets-Sheet l my w S nger N Wm .42. M m m a 6 4 a, A mm 7- m5 w/ r I mm e f-l MN 0. r 86 a. my .l 5 9.8 M .mfl M. e aw n WM s w m h .n dm Wm a m mm 0 EM 9 0 0 0 0 0 .x m m m w w M m m H QQQ G Hardening Temperafure F //7/: of Temperature INVENTORS Harry Tanczyn 0 6a eron Perry BY ATTORNEY Oct. 6, 1964 D c. PERRY ETAL 3,151,978

HEAT HARDENABLE CHROMIUM-NICKEL-ALUMINUM STEEL Filed Dec. 30, 1960 3 Sheets-Sheet 2 [.76 2 Snart Time Elevated Temperature Tensile Strength of Gr- /V/'- Ma-A/ Stainless Steel l000psi.

260 Preferred Heat Treatment Alternate Heat 7reatment T ens/7e Strength IOQOps/ Test T emperature- "F INYENTORS Harry Tanczyn 0 Cameron Perry TTORNEY Oct. 6, 1964 Filed Dec. 30, 1960 FB /rife 0r 1 440+ SH- 24/ D c. PERRY ETAL 3,151,978

HEAT HARDENABLE CHROMIUM-NICKEL-ALUMINUM STEEL 5 Sheets-Sheet 5 FIG. 3

E ffec/ of Chemical Bar/once of 67- A i-Ma Al Stainless S/ee/ on Tens/7e .Sfrengf/I in /,0Q7ps/'.

INVENTORS Harry Tanczyn 0 Came/0n rry BY 1 J ATTORNEY United States Patent Office 3,151 ,978 Patented Oct. 6, 1964 3,151,978 HEAT HARDENABLE CHROMIUM-NICKEL- ALUMINUIW STEEL D Cameron Perry, Middletown, Ohio, and Harry Tanczyn,

Baltimore, Md, assignors to Armco Steel Corporation,

a corporation of Ohio Filed Dec. 30, 1960, Ser. No. 79,922 14 Claims. (Cl. 75-124) This application for patent is a continuation-in-part of our copending application Serial No. 842,800, filed September 28, 1959, entitled Heat Hardenable Chromium- Nickel-Aluminum Steel, which in turn is a continuationin-part of our then copending application Ser. No. 706,471, filed December 31, 1957 and entitled Stainless Steel and Method, now abandonded, and our invention relates to the chromium-nickel steels and more particularly to the heat-hardenable chromium-nickel-aluminum steels and a method of hardening the same, and to the resultant hardened steel.

One or" the objects of our invention is the provision of a heat-hardenable chromium-nickel steel which possesses good hot-working properties and yet which readily may be hardened by heat-treating methods to achieve great strength at room temperatures and great strength at high temperatures.

Another object is the provision of a chromium-nickelaluminum steel which may be converted into plate, sheet, strip, bars, rods, wire, tubes and special shapes through rolling, drawing, piercing and extrusion methods; which steel will not objectionably harden in shipment in Winter weather; and which upon receipt by a customer fabricator may be formed and fabricated into a host of articles, which may be hardened either before or after fabrication by heat-treatment.

A further object of our invention is the provision of a simple, direct, reliable and thoroughly practical method of producing heat-hardened chromium-nickel-aluminum steel of great strength.

And another object of our invention is the provision of a heat-hardened chromium-nickel-aluminum stainless steel and various articles and products fashioned of the same with great strength and good surface appearance.

Other objects of our invention in part will be obvious and in part pointed out in the description which follows.

Accordingly, our invention consists of the combination of elements, composition of ingredients and in the various operational steps and the relation of each of the same to one or more of the others, and to the combination of composition and operational steps as described herein, the scope of the useful application of which is indicated in the claims at .the end of this specification.

In order to gain a better understanding of certain features of our invention it may be noted at this point that although the usual grades of chromium-nickel stainless steel are not considered to be hardenable by heat-treating methods, there are available chromium-nickel stainless steels containing the further ingredients titanium and/or columbium which respond to heat-hardening methods. With a balanced composition of chromium, nickel, titanium and/ or columbium together with carbon, there is had a steel possessing substantial hardness in the final heathardened state. These steels rely on the carbide-forming characteristics of the titanium and the columbium, in fact, on the circumstance that both are stronger carbide-formers than is chromium. It is generally known, however, that columbium is an expensive addition and is not readily available. Also it is known that titanium is inclined to produce dirty metal, requiring special steps in the melting operation to produce a quality steel. But with all this, the strength had in the heat-hardened condition leaves much to be desired.

More recently it is to be noted that a series of chromiumnickel-aluminum steels are finding great favor in the art. We refer, for example, to the steels described and claimed in the US. Patents 2,505,762; 2,505,763; 2,505,764 and 2,506,558, all of which issued to George N. Goller on May 2, 1950. In the steels of those patents there are employed critical amounts of the ingredients chromium, nickel and aluminum to achieve desired workability in the annealed condition, this in combination with desired hardness and strength in the heat-hardened condition. In the steels of those patents there additionally may be included small amounts of manganese and silicon, sulphur and phosphorus, as well as small amounts of molybdenum, this latter to enhance the corrosion-resistance of the steel. With the balanced composition of the steels of the patents and the special heat-hardening treatment there are achieved ultimate tensile strengths somewhat in excess of 200,000 p.s.i. with 0.2% ield strengths closely approaching the 200,000 p.s.i. figure (see the tables appearing at the foot of col. 6 of the Patents 2,505,763 and 2,505,764). The workability of the steels of the patents in the annealed condition is good, the hardness in the annealed condition being somewhat less than Rockwell B90. Some of these steels, however, require a special stabilizing treatment to prevent an unwanted hardening during shipment in winter Weather.

An object of our invention is the provision of a chromium nickel aluminum steel possessing good hot-working characteristics and yet which readily may be hardened by simple heat-hardening methods to achieve even greater strength in the heat-hardened condition than the heretofore chromium-nickel-aluminum stainless steels, all without objectionable unwanted hardening in shipment.

1n the accompanying drawing illustrating certain features of our invention:

FIG. 1 is a graph illustrating the efiect of the hardening temperature on the strength of our steel;

FIG. 2 is a graph illustrating the elevated temperature strength of our steel; and

FIG. 3 is a chart on which there is indicated the strength of our steel as it is affected by the chemical balance of its austenite-forming and ferrite-forming constituents.

In the practice of our invention we find that by closely correlating the ingredients chromium and nickel with the further ingredients aluminum and molybdenum we obtain a steel possessing good hot-working characteristics. The steel is in every sense critical. And we find that .by proper heat-treatment from the annealed condition there is achieved great strength and hardness without, however, inadvertently hardening the steel as a result of coldweather shipment in the annealed condition.

The steel of our invention essentially consists of about 4% to 14.5% chromium, 6% to 12% nickel, 4% to 14% molybdenum, .5 to 2% aluminum, carbon not exceeding about .20%, and remainder substantially all iron. Ordinarily, the manganese content does not exceed 2% and the silicon content does not exceed 1%. Where desired, however, manganese can be substituted for nickel on the basis of 2 for 1. But in this substitution we find it necessary that there must be at least 3.5 nickel in order to achieve the desired ultimate hardness. The actual nickel content, therefore, is at least 3.5% with the remaining nickel requirement supplied by manganese over and above the 2% manganese commonly present as a maximum, this on a 2 for 1 basis. Thus, for the maximum nickel content of 12%, there may be an actual nickel content of 3.5% with a substituted nickel content of 8.5%, represented by an addition of 17% manganese, with total manganese content, then, of 19% as a maximum.

The ingredient silicon may be substituted for a small portion of the chromium content on a 1 for 1 basis for the purpose of increasing the heat-scale resistance of the steel. The extent of this substitution, however, is limited to replacing no more than 2% of the chromium with silicon which, with the 1% maximum silicon content ordinarily present, gives a total silicon content of 3% as a maximum.

In our steel we find that molybdenum may be replaced by tungsten on a 1 for 1 basis, this up to a total tungsten content of 6%. The molybdenum content and also the tungsten content of our steel may be partially replaced by vanadium on a 1 for 1 basis, this up to a total vanadium content of 2%.

Where desired, we find that boron up to the amount of .010% may be added to improve the hot-workability of the steel, without loss of ductility or undue brittleness in the final heat-hardened condition. And, where desired, there are added titanium and/ or zirconium up to 0.10%. Also, where desired, there is added colurnbium up to 0.50%. The additions of titanium, zirconium and columbium have the effect of tying up the small amount of carbon present in the steel, with the result that the hardness in the annealed condition is lowered significantly and formability correspondingly improved.

Our steel ordinarily, however, consists essentially of 4% to 14.5% chromium, 6% to 12% nickel, 4% to 14% molybdenum, .5 to 2% aluminum, the carbon content not exceeding .20%, manganese up to 2%, silicon up to 1%, phosphorus and sulphur each not exceeding .05%, with remainder iron. One preferred steel analyzes approximately 8% to 14.5 chromium, 6% to 10% nickel, 4% to 8% molybdenum, .5% to 2% aluminum, carbon not exceeding 0.20%, and remainder substantially all iron. This steel is stainless, is readily workable in the annealed condition, and'is hardenable by heat-treating methods to great strength.

A preferred range of composition for the stainless steels is about 11.25% to 12.75% chromium, 6.5% to 7.5% nickel, 5.5% to 6.5 molybdenum, .8% to 1.4% aluminum, .06% to .10% carbon, and remainder substantially all iron. In this steel the manganese content is up to 2% as a maximum, the silicon content up to 1% and phosphorus and sulphur each up to .05%. A particular example consists of about 12% chromium, 7% nickel, 6% molybdenum, .8% to 1.4% aluminum, carbon not exceeding about 0.20%, and remainder substantially all iron.

Another preferred composition of stainless steel in accordance with our invention essentially consists of 10.75% to 12.00% chromium, 7.5 to 8.5% nickel, 5.5% to 6.5 molybdenum, .8% to 1.4% aluminum, with carbon up to 0.20%, and remainder substantially all iron. Here again the manganese is up to 2%, the silicon up to 1% and phosphorus and sulphur each up to .05%. A further preferred steel essentially consists of about 10% to 11.5% chromium, 8% to 9.5% nickel, to 7% molybdenum, .5 to 1.5 aluminum, and remainder substantially all iron. 7 Minor amounts of carbon, manganese, silicon, sulphur and phosphorus, of course, are present as noted. A

specific preferred steel analyzes about 11% chromium, 9% nickel, 6% molybdenum, 1% aluminum, and remainder substantially all iron. This steel as well as the further preferred steel noted above, is almost entirely free of delta ferrite, the delta ferrite content being less than about 5% by volume and usually less than 2%. The further preferred steel, and specific preference, is especially suited to the production of bars, billets, forgings, plates and flat-rolled products of heavy section.

In the steels of our invention we preferably maintain chromium on the low side where the molybdenum content is to be on the high side, and conversely, where the molybdenum is to be on the low side, the chromium is maintained on the high side. Thus one of our stainless steels consists essentially of about 13% to 14% chromium, 6.5 to 7.5 nickel, .8% to 1.40% aluminum, about 4% molybdenum, with remainder substantially all iron. Another essentially consists of about 11.25% to 12.75% chromium, 6.5% to 7.5% nickel, .8% to 1.40% aluminum, about 6% molybdenum, and remainder substantially all iron. A further stainless steel essentially consists of 8% to 10% chromium, 8% to 9% nickel, .8% to 1.40% aluminum, about 8% molybdenum, and remainder substantially all iron. More particularly, the steel analyzes about 9% chromium, 9% nickel, 8% molybdenum, .8% to 1.4% aluminum, and remainder substantially all iron.

Some further steels in accordance with our invention essentially consist of 4% to 8% chromium, 7.5 to 12% nickel, 8% to 14% molybdenum, .5% to 2% aluminum, carbon not exceeding 0.20%, and remainder substantially all iron. These steels although not properly considered as stainless steels, are readily workable in the annealed condition and are hardenable by heat-treating methods to great strength. One such 'steel, more particularly analyzes about 8% chromium, 9% nickel, 10% molybdenum, .8% to 1.4% aluminum, and remainder substantially all iron. Another analyzes about 5% chromium, 12% nickel, 12% molybdenum, .8% to 1.4% aluminum, and remainder substantially all iron.

A further steel, one which is hardenable by a single precipitation-hardening heat-treatment from the annealed or solution-treated condition, essentially consists of 4% to 9% chromium, 6% to 10% nickel, 4% to 8% molybdenum, .5% to 2% aluminum, carbon .10% max., and remainder substantially all iron. In this steel the sum of the chromium and molybdenum contents should be less than 14% in order to prevent the development of delta-ferrite with consequent loss of transverse strength ductility. And for the same reason the nickel content should not be less than 6%.

One preferred single-treatment steel essentially consists of about 5.5% to 6.5% chromium, 8.5% to 9.5% nickel, 5.5% to 6.5% molybdenum, .8% to 1.4% aluminum, carbon .08% max., and remainder substantially all iron. Another analyzes about 4% to 5% chromium, 7.5 to 8.5% nickel, 5.5% to 6.5% molybdenum, .8% to 1.4% aluminum, carbon .08% max., and remainder substantially all iron. A further preferred single-treatment steel analyzes about 7% to 8% chromium, 7.5% to 8.5% nickel, 5.5 to 6.5% molybdenum, .8% to 1.4% aluminum, carbon .08% max., and remainder substantially all iron.

The steels of our invention lend themselves to conversion by known rolling, drawing, piercing and extruding operations, from the ingot and billet into various commercial forms such as plate, sheet, strip, bars, rods, wire, tubes and various special shapes. And we find that they may be shipped to a customer in the annealed condltion (annealed, for example 1950 to 2050 F.) without encountering an undesired transformation, even in extreme cold weather.

The steels when received by the customer fabricator readily may be fabricated through known techniques such as bending, punching, machining, and the like, and by welding, riveting, etc. into a variety of products which subsequently are hardened and strengthened by heat-trea ment. Our steels, where desired, may be made into castings which are machined to specification. The heattreating operation, or one or more of the steps thereof, for both wrought and cast products, may be performed prior to fabrication. In fact, one or more of the conditioning treatment, cold treatment and final hardening may be done at the steel plant prior to shipment when there is little or no drastic shaping required of the fabricator.

The steels of our invention preferably are heat-hardened by first conditioning the annealed steel at a temperature of about 1200 to 1800 F., or even 1000 to 1800 F., and cooling, this to facilitate subsequent hardening treatment. Then cold-treating the steel by holding the same at a temperature of 150 to 320 F., or 150 to l00 F., preferably 60 to -320 F., or more particularly 60 to 100 F. And finally heat-hardening by reheating the steel to a temperature of 800 to 1150 F. or even 700 to 1200 F. Actually, we prefer to condition the annealed steel at a temperature of 1300 to 1650 F., more particularly at 1400 F. The time of treatment usually ranges from about 10 minutes up to an hour or more, preferably about 2 hours at preferred temperature. We find that no particular benefit is had through prolonged conditioning treatment beyond about 2 hours.

While we may cold-treat our steel at a temperature anywhere from 60 to 320 F. for periods of time ranging from 2 hours to 24 hours or more, we preferably refrigerate the steel at a temperature of about 100 F u this for a period of about 8 hours, or 25 F. for 2 hours following the preferred conditioning treatment of 1400 F. for 2 hours.

In hardening the cold-treated steel we preferably employ a temperature of 950 to 1050 F. for a period of about 1 hour. With the conditioning treatment at 1400 F. for 1 hour and cooling followed by refrigeration at 100 F. for 8 hours, we prefer to heat-harden the steel by a treatment at about 1000 F. for a period of 2 hours.

The effect of the conditioning temperature and time on a typical steel of our invention following annealing is given in Tables 1(a) and 1(1)) below, the analyses of the steel, and two additional examples which will be referred to hereinafter, being given in the former, and the mechanical properties after final heat-treatment following the conditioning treatment being given in the latter.

TABLE 1(a) Chemical Analyses of Cr-Ni-Mo-Al Stainless Steel Effect of Conditioning Temperature and Time on Room Temperature Mechanical Properties of the Example R1078 Annealed Steel of Table 1(a) Conditioning Time=10 mins. Conditioning Time=60 mins.

00110. 0.2% T.S Per- 0.2% T.S., Per- Temp., Y.S., p.s.i cent R Y.S., p.s.i. cent R 10 min. p.s.i E in p.s.i. E in From the data presented in Tables 1(a) and 1(1)) above it will be noted that best results in terms of ultimate tensile strength and .2% ield strength are had Where the previously annealed steel is conditioned at a temperature between 1300 and 1600 F., particularly about 1400 F. A treating time of 1 hour, it will be noted, gives somewhat better results than the shorter treating time of 10 minutes.

The effect of the temperature of the cold-treatment and the time of the cold-treatment are illustrated by the data presened in Table I(c) below:

TABLE I(c) Efiect of Temperature of Cold-Treatment on Room Temperature Mechanical Properties of Steel R1078 0] Table 1(a) Hardened for 1 Hour at 950 F.

Gold 0.2% 00nd. Temp. m Temp.in Y.S., T.S Percent Ro F., 10 minutes F., 8 psi. p.s.i. E in 2 hours Although the mechanical properties for steels hardened following cold-treatment at l00 F. and 50 F. are about the same, the difference is in favor of the lower temperature treatment. Treatment at either of these temperatures, however, is but slightly better than treatment at 0 F. The -100 F. cold-treatment is preferred. Actually, we find that as a practical matter refrigeration at a temperature of 25 F. is adequate.

While neither the conditioning temperature of our steel nor the temperature of cold-treatment is highly critical, we do find that the temperature of the final hardening treatment is most critical. The critical character of the final hardening heat-treatment in terms of its effect upon the mechanical properties at room temperature is illustrated by the data presented in Table 1(d) below:

TABLE I(d) Efiect 0f Hardening Temperature on the Room Temperature Mechanical Properties of Conditioned and Cold- Treated Example R1078 of Table 1(a) Hard. Temp. F. 0.2% Y.S., T.S.,p.s.i. Percent R (1 hr. at temp.) p.s.i. E in 2" It will be seen from the data of Table I(d), particularly the tensile strength and yield strength figures, that best results are achieved where the final hardening treatment is conducted within the critical range of 950 to 1050 F. Within this range tensile strength values in excess of 25 0,- 000 p.s.i. are had with yield strength which generally averages in excess of 220,000 p.s.i. The critical natural of the final hardening treatment is forcefully revealed by the curves for tensile strength and yield strength as against the hardening temperature which is given in FIG. 1 of the drawings. These curves support the gen- 7 eral acceptable range of 800 to 1150" F. with the more critical range of 950 to 1050 F.

While we prefer a hardening heat-treatment consisting of the three steps of conditioning the annealed steel, then cold-treating for a period of time, and finally heat-hardening the same, beneficial results with many of the advantages noted are archieved with other heat-treatments. For example, we find that good results are had by merely cold-treating the steel at a temperature of about 100 F. for 8 hours from previously treating at a temperature of 1750 F., with or Without annealing, and then hardening the steel by heat-treatment at a temperature of some 800 to 1150 F. Mechanical properties had at room temperature for a sample of our steel analyzing 13.41% chromium, 7.35% nickel, 4.10% molybdenum, 1.20% aluminum, .095 carbon, .81% manganese, .64% silicon, 012% phosphorus, 020% sulphur, and remainder iron, when cold-treated at 100 F. from a previous treatment at 1750 F. for minutes and then hardened by heat-treatment at temperatures of 750, 825 and 950 F., are given in Table II below:

TABLE II Effect of Final Hardening Temperature 011 Room Temperature Mechanical Properties of Cold-Treated and Heat-Hardened Cr-Ni-Mo-Al Stainless Steel Strip of .050" Gage It will be noted from the data presented in Table II that best results in terms of tensile strengths and yield strengths are had with the higher temperatures of final hardening treatment.

Moreover, it will be noted that the resutsare comparable with the tensile and yield strengths had with the preferred treatment of conditioning, cold-treating and'final heat-hardening, as presented in Table I(d).

Another heat-treatment which gives some benefits of our invention is one involving conditioning the steel at 1400 F. for 1% hrs. or more, cooling the same to room temperature (60 F.) within an hour, and then reheating to about 1050 F. for 1% hours. The mechanical prop erties had with the sample of our steel analyzing as above, whose properties are given in Table II, are set forth in Table III below:

TABLE III Mechanical Properties of Conditioned and Heat-Hardened Cr-Ni-Mo-Al Stainless Steel Strip of .050" Gage 0.2% T.S Elong. Rockwell Condition Y.S., p.s.i percent Hardness p.s.i in 2 Hard. at 1,050 F 215,000 227,400 7 11048.5.

'8 are given in Table 1V('b) below, the chemical analyses of the several steels being given in Table IV(a):

TABLE IV(a) Chemical Analyses of Cr-Ni-Mo-Al Stainless Steels Chemical Balance Heat 0 Factor Fer.

R1419 R1409 R 1407 TABLE IV(b) Short Time Elevated Temperature Tensile Properties on Cr-Ni-Mo-Al Stainless Steel (1) ANNEALEDl-OONDITIONED AT 1400 F. COLD- TREATED F.+HEAT-HARDENED 1000 F.

Percent Aus.

Ein2" Heat (2) ANNEALED-I-TREATED AT 1,750 F.+ OGLD- TREATED 100 F.+HEAT-HARDENED 1,000 F.

HH potoqoacn can It will be seen by simple comparison of the data of Parts (1) and (2) of the Table IV(b) that the short time elevated temperature strength of our steel with preferred heat-treatment is significantly superior to the strength had when given the alternate form of heat-treatment. This comparison is illustrated in FIG. 2 of the drawings.

Another preferred treatment for the steels of our invention, following the annealing at 1900 to 2050 F., is a conditioning at 1000 F. to 1200 F. and cooling. Following this, the steel is refrigerated at 30 to F. And then it is hardened by reheating at 700 to 1150 F., especially about 1000 to 1050 F.

Hardening of our steels also is achieved by drastic coldrolling, or other cold-working, that is, to the extent of at least 25% and preferably about 60% or more, followed by precipitation-hardening heat-treatment. In a steel analyzing about 11% chromium, 8% nickel, 6% molyb- TABLE X(a) Chemical Analysis Specific Chromium-Nickel- Alnmz'num Stainless Steel MI]: 1? S Si N Al Mechanical Properties of Specific Steel of Table X (a) CH1050=Cold rolled 60% hardened at 1050 F. for 1 hour.

All tests made in longitudinal direction.

The great tensile strength of our cold-rolled and precipitation-hardened steel, this well exceeding 300,000 p.s.i., is to be especially noted, as well as its excellent yield strength, a value approaching 300,000 psi.

The general effect of the chemical balance of our steel, that is, the balance between the austenite-forming ingredients figured at 25 the carbon content-l-Vz the manganese content-t-the nickel content, as against the ferriteforming ingredients figured at the sum of the chromium, molybdenum, md silicon contents-l-twice the aluminum content shows that the greatest strength is achieved where the sum of the austenite-forming ingredients is on the high side and the sum of the ferrite-forming ingredients is on the low side. The mechanical properties for a series of examples of our steel of differing chemical analyses is given in Table V(b) below, the analyses of the samples being given in Table V(a), this including the samples noted in Table 1V(a):

TABLE V(ll) Chemical Analyses of 127-6 Cr-Ni-Mo-Al S taznless Steels Chemical Balance Factors Heat C N111 S1 C1 Ni Al DIO Aus Fer.

10 TABLE v 5 PH12-7-6 Mechanical Properties of Cr-Ni-Mo-Al Stainless Steel Conditioned, Refrigerated and Heat-Hardened 1 0.2% Y.S., Percent E Rockwell 0 Heat No. psi T.S., 11.6.1. Hardness 1 1,400 F. for 1 hr.; refrigerate 8 hrs. at -100 F.; harden at 1,000D F. for 111.

It will be noted from the results given in Tables V(a) and V(b) that the steels of the high tensile strengths are those in which the sum of the austenite-forming ingredicats is high and the sum of the ferrite-forming ingredicats is low. This is graphically illustrated in FIG. 3 of the drawing.

A further series of preferred steels according to our invention is given in Table V1(a) below, with cold treatment test data, delta ferrite and balance between austenite-forming ingredients and ferrite-forming ingredients given in Table V1(b), and with corresponding mechanical properties in hardened condition given in Tables V1(c) and VIM).

TABLE V1(a) Chemical Analyses of 11-8-6 Cr-Ni-Mo-Al Stainless Steels Heat No. 0 Mn Si Cr Ni Me Al 066 57 47 11. 27 7. 77 5. 88 l. 15 069 59 49 11. 83 7. 72 6. 00 1. 15 071 57 48 10. 53 8. 90 5. 79 1. 22 076 44 46 11. 26 8. 05 5. 1. 16 075 37 37 11. 17 8. 60 5. 94 1. 17 076 44 39 11. O5 8. 99 6. 00 1. 13 077 44 40 9. 76 9. 06 6. 29 l. 09 078 40 41 11. 74 8. 11 6. 15 1. 14 077 47 41 9. 72 9. 51 6. 23 1. 12 083 38 41 11. 78 8. 58 6. 21 1. 03 081 45 44 11. 12 8. 47 6. 07 99 082 43 39 11. 08 8. 94 6. 00 1. 13 080 44 46 10. 47 8. 99 5. 99 1. 24 080 49 38 11. 20 8. 23 6. 02 1. 27

TABLE VI(b) Cold T rent Test Data, Delta Ferrite, and Chemical Bal ance 0] 11-8-6 Cr-Ni-Mo-Al Stainless Steels Hardness Hardness Chemical Balance Annealed Cold Percent Heat No. Rock- Treat Delta well B Rock- Ferrite Ans. Fer

well B 11 From Table VI(b) it is noted that the amount of delta ferrite in our steels is less than 12% by volume (usually less than and frequently under 5%. The hardness in the annealed condition usually is less than Rockwell 12 vstantially'all ironyand 4% to 6% chromium, 10% to 12% nickel, 12% molybdenum, .8% to 1.4% aluminum, and remainder substantially all iron), there are given below several examples for each group in Table VII(a), the

B90. And this hardness ordinarily is not increased by 5 corresponding mechanical properties being given in Tatreatment at temperatures reached in cold weather shipble VII(b): ment (0 F.); the occasional increase is not over 1 or 2 Pomts' TABLE Vll(a) TABLE VI( 10 Chemical Analyses of Three Groups of Cr-Ni-Mo-Al Steels (About 8%, 10% and 12% Molybdenum) With Mechanical Properties of l186 Cr-Ni-Mo-Al Stainless I di atio of Chemical Balance Steel Conditioned, Cold-Treated and H eat-Hardened 1 15 CBhelmical 0.27 Y S Percent E Rockwell O a ance Heat p .s.i 2" Hardness Heat 0 Mn 81 Cr Ni M0 .41 Factors 260,000 279,100 4 54 Aus. Fer. 248, 800 271, 000 4 53 251,800 281,500 4 54.5 255,600 282,000 4 54 R2233 083 .64 .66 9.56 9.43 8.00 1.17 11.825 20.56 259,200 282,200 4 54 32391-- 070 .51 .52 8.89 8.37 7.87 1.14 10. 373 19.56 229,000 257,400 4 53 112392-- 073 .54 .58 8.93 8.90 8.03 1.15 10.995 19.84 255,000 283,600 6 54.5 112394-- 675 .50 .52 8.23 3.40 8.03 1.13 10.525 19.14 196,100 260,000 4 51.5 182395.. 078 .49 .56 8.36 9. 00 8.10 1.20 11.195 19. 42 250,000 275,200 5 53.5 R2237 070 .67 .73 7.69 9.47 10.00 1.12 11.555 20.66 235,000 262,100 6 53 R2397" 060 .54 .56 7.03 9.38 10.20 1.14 11.30 20.07 255,000 278,500 5 54 R2400 076 .47 .48 7.85 8.89 10.13 1.16 11.025 20.78 231,000 259,000 6 52.5 R2243 .080 .67 .82 6.12 9.84 12.30 1.14 12.175 21.54 232,500 265,600 9 53 R2401" .058 .51 .61 4.51 11.86 12.14 1.23 13.815 19. 72 263,600 284,000 4 54.5 182404-- .081 .49 .58 3.72 10. 82 11.93 1.13 13.09 18.49

1 (gondition 1400 F. for 2 hrs. refrigerate 25 F. for 2 hrs.; and harden 1000 iorzhrs The amount of delta-ferrite for the above steels is less than 12% by volume, and for the second and third groups TABLE M) of steels usuall amounts to some 3% to 5% by volume.

Mechanical Properties of ll-8-6 Cr-Ni-Mo-Al Stainless Steel Conditioned, Cold-Treated and Heat-Hardened TABLE VH0) With Furnace Coolin 1 g Mechanical Properties of the Three Groups of Cr-N1-M0- Al Steels (8%, 10% and 12% Molybdenum) of Table Heat No T.S., p.s.i l E fi ggg VII (a) as Conditioned, Cold-Treated and Heat-Hard- 40 ened 1 267, 400 290, 300 4 247, 200 280, 200 5 55 259, 000 290, 900 3 56 Heat N0. 0 2% Y S T.S., p.s.i. Percent E Rockwell G 270, 000 290, 000 4 55. 5 p.s.i 2 Hardness 267, 600 289, 000 4 55 241,400 268, 000 5 53 257,000 290,600 6 55 199,000 271,500 4 53 216,000 265,200 5 53 280, 000 313, 000 2 57 266, 000 286, 600 4 55 238,000 315, 500 2 57 244, 100 270, 000 3 54. 5 271, 000 304, 200 1 57 260, 000 286, 600 4 55. 5 291, 000 298, 000 58 230, 200 265, 000 7 53 274, 000 307, 000 2 57 237,200 273, 500 5 54 276, 000 296,000 1 57 269,200 294, 000 5 56 260, 000 287,400 1 56 290,000 296,500 0 5 60 296, 000 300, 000 1 58 1 Condition 1400 F.1or 2 hrs.; refrigerate 25 F. for 2 hrs.; and harden 307, 000 326, 000 l 60 1000 F. for 2 hrs.; furnace cool to 950 F., hold 3 hrs.; furnace cool to 900 F., hold for 3 hrs.; and air cool.

In general, it is seen from Table VI(c) and again from Table VI(d) that greatest strength is had in the steels of maximum austenitic content and minimum ferrite content, as given for the steels in Table VI(a). Moreover, by comparing Table VI(c) with Table V(b) it is seen that the steels of the most preferred composition, generally referred to as 11-8-6 Cr-Ni-Mo-Al stainless steel,

possess even greater strength than those of the 1276 composition. Also, by comparing the mechanical properties of the steels hardened by the two different methods, it is seen that best results are achieved with the hardening treatment employing the delayed furnace cooling (that is, the treatment of Table Vl(d)). 1'

As specific example of our steels of the high molybdenum contents, that is 8%, 10% and 12% (the steels respectively analyzing about 8% to 10% chromium, 8.5% to 9.5% nickel, 8% molybdenum, .8% to 1.4% a1umi num, and remainder substantially all iron for one group; about 7% to 8% chromium, 9% to 10% nickel, 10% molybdenum, .8% to 1.4% aluminum, and remainder sub- 1 Samples in the form of strip conditioned 1400 F.1or 2 hrs.; refrigerated F. for 8 hrs. hardened 1000 F. for 2 hrs.

Comparable mechanical properties are had where the conditioning treatment is had at 1200 F. for 2 hours. Also where it is had at 1700 F. for some 10 minutes to 2 hours. Actually, in the lower chromium, higher molybdenum grades, it appears that the greatest tensile strength is bad with the 1700 F. conditioning treatment followed by the refrigeration at 100 F. and hardening at 1000 F., the tensile strength of the 12% molybdenum grade VHI(a) below, with mechanical properties in heathardened condition in Table VIII(b) below:

TABLE VIIl(a) Chemical Analyses of 4 Cr-Ni-Al Heat-Hardenable Stainless Steels Heat No C Mn P 5 s1 Or Ni Mo A1 W V TABLE Vlll(b) 1 may be conducted at the broader temperature range of Mechanical Properties of the Steels of Table VH1(a) Following Treatment at 1400 F., C0ld-Treating at ]00 F. for 8 Hrs. and Heat-Hardening at 1050 F. for 1 Hour Heat No. Section U.1.S., 2% Y.S., Percent Rockwell p.s.i. psi. E Hardness 230, 800 4. 0 C5l.5. 227, 000 9. 3 C50. 221, 100 10. 7 C51. 206, 300 10. 7 C50.

It will be noted from the Tables VlII(a) and Vlll(b) given above that the ultimate tensile strength of about 250,000 p.s.i. with a yield strength of about 220,000 p.s.i. is had with the steels containing molybdenum and/or tungsten with strengths somewhat less than these figures for the steel containing all three of molybdenum, tungsten and vanadium. The hardness in all cases is Rockwell C50 or better. Certain further benefits are achieved by including in the composition of our steel the ingredients titanium or zirconium up to about 0.10%. This addition improves the weldability, assuring soundness and freedom from pin-holes adjacent the Weld. Moreover, on occasion we include up to about 0.50% columbium in our steel for the purpose of improving its toughness.

As specifically illustrative of the single-treatment steels of our invention, reference is made to three examples given in Table lX(a) below; with mechanical properties in the annealed condition 1900" F. for /2 hr. and oilquench) and precipitation-hardened condition (above anneal+1050 F. for 1 hour and air-cool) being given in Table IX(b):

TABLE IX(a) Chemical Analyses of 3 Cr-Ni-Al Hardenable Single- Treatment Steels Heat No. 0 Mn P S Si Cr Ni Mo Al R2711 O27 33 018 .011 44 4. 05 8.08 6. 42 1. 09 R2707. 037 39 020 029 19 6. 23 9. 55 5. 73 1. 22 031 33 O18 O12 47 7. 60 8. 19 6. 37 1. 11

TABLE L(( 1;)

Mechanical Properties o the Steels of Table IX (a) in Annealed Condition (1900 F. /2 Hr.Oil-Qnench) and in Final Heat-Hardened Condition (Anneal +1050 F.1 Hr.Air-Cool) While solution-treatment at a temperature of 1900 F. and precipitationhardening at 1050 F. are employed for 1500 to 2050 F. and quenched in air, oil or Water. In this condition the steel is martensitic. And that hardening may be had by treatment at 800 to 1150" F. and cooling.

It will be seen that we provide in our invention a chromium-Iiickel-aluminum-molybdenum steel of great strength. It is a steel which is readily workable and formable in the annealed condition and yet which is hardened to ultimate tensile strengths of 270,000 psi. and more, with yield strengths of about 240,000 p.s.i. or more.

Note that in the single-treatment steels there are had low hardness and good ductility in the annealed condition in combination with great tensile strength and high yield strength in the heat-hardened condition. Note also that ductility in the hardened condition as reflected by the elongation and reduction in area is good and favorably compare with that of the steel hardened by double-treatment methods. These steels are particularly low in delta-ferrite content, this seldom exceeding 2% by volume, and enjoy good transverse properties, properties which are particularly important Where multi-axial stresses are encountered in actual use. The outside casings of missiles, rockets and supersonic aircraft conveniently are fashioned of our steel because of its isotropic characteristics.

Thus it will be seen that We achieve the various objects of our invention hereinbefore set forth, together with many practical advantages in terms of workability and formability in one condition of heat-treatment, and great strengths and hardness in another.

As many possible embodiments may be made of our invention and since many changes may be made in the embodiments set forth above, it will be understood that all matter described herein or shown in the accompanying drawings is to be interpreted as illustrative and not by way of limitation.

We claim as our invention:

1. A chromium-nickel-aluminum steel which is substantially fully austenitic at annealing temperatures and which is hardenable by heat-treatment, said steel consisting essentially of about: 4% to 14.5% chromium, 6% to 12% nickel, .5% to 2% aluminum, 4% to 14% molybdenum, manganese up to 2%, silicon up to 1%, carbon up to 20%, up to about 0.10% of the group consisting of titanium and zirconium, up to about 0.50% columbium, boron up to 0.010%, and remainder substantially all iron, with additional manganese, in excess of said 2%, partially substituted for said nickel on the basis of 2 for 1, the actual nickel content amounting to at least 3.5%, and the total manganese content up to 19%, additional silicon substituted for chromium on the basis of 1 for 1 up to a total silicon content of 3%, tungsten substituted for molybdenum on a 1 for 1 basis up to a total tungsten content of 6%, and vanadium substituted for molybdenum and tungsten on a 1 for 1 basis up to a total vanadium content of 2%.

2. A chromium-nickel-aluminum stainless steel which is hardenable by heat-treatment from a substantially fully austenitic condition, said steel consisting essentially of about: 8% to 14.5% chromium, 6% to 10% nickel,

15 4% to 8% molybdenum, .5% to 2% aluminum, carbon not exceeding 020%, and remainder substantially all 3. A chromium-nickel-aluminum steel which is hardenable by heat-treatment, said steel consisting essentially of about: 4% to 8% chromium, 7.5% to 12% nickel, 8% to 14% molybdenum, .5% to 2% aluminum, carbon not exceeding 0.20%, and remainder substantiallyall lI'OIl.

4. A chromium-nickel-aluminum steel which is substantially fully austenitic at annealing temperatures and which is hardenable by heat-treatment, said steel consisting essentially of about: 4% to 9% chromium, 6% to nickel, 4% to 8% molybdenum, .5% to 2% aluminum, carbon .10% max, and remainder substantially all iron.

5. A chromium-nickel-aluminum steel which is substantially free of delta-ferrite and hardenable by single heat-treatment from the solution-treated condition, said steel consisting essentially of about: 4% to 9% chromium, 6% to 10% nickel, 4% to 8% molybdenum, with the sum of the chromium and molybdenum contents being less than 14%, .5% to 2% aluminum, carbon .10% max., and remainder substantially all iron.

6. A chromium-nickel-aluminum stainless steel which is hardenable by heat-treatment from a substantially fully austenitic condition, said steel consisting essentially of about: 10% to 11.5% chromium, 8% to 9.5% nickel, .5 to 1.5% aluminum, 5% to 7% molybdenum, carbon not exceeding 0.20%, and remainder substantially all 7. A chromium-nickel-aluminum stainless steel which is hardenable by heat-treatment from a substantially fully austenitic condition, said steel consisting essentially of about: 11.25% to 12.75% chromium, 6.5% to 7.5% nickel, .8% to 1.4% aluminum, 5.5% to 6.5% molybdenum, .06% to .10% carbon, and remainder substantially all iron.

8. A chromium-nickel-alurninum stainless steel which is hardenable by heat-treatment from a substantially fully austenitic condition, said steel consisting essentially of about 12% chromium, about 7% nickel, about 6% tungsten, .8% to 1.40% aluminum, carbon not exceeding .20%, and remainder substantially all iron.

9. A chromium-nickel-aluminum stainless steel which is hardenable by heat-treatment from a substantially fully austenitic condition, said steel consisting essentially of about 12%-"chromium, about 7% nickel, about 2% molybdenum, about-3% tungsten, .8% to 1.40% aluminum, carbon not exceeding .20%, and remainder substantially all iron.

10. A chromium-nickel-alurninum stainless steel which is hardenable by heat-treatment from a substantially fully austenitic condition, said steel consisting essentially of about 12% chromium, about 7% nickel, about 2% 16 molybdenum, about 2% tungsten, about 1% vanadium, .8% to 1.4% aluminum, carbon not exceeding .20%, and remainder substantially all iron.

11. A heat-hardened chromium-nickel-aluminum steel of great strength at room temperatures and at high temperatures substantially free of delta-ferrite and consisting essentially of about: 4% to 14.5% chromium, 6% to 12% nickel, .5% to 2% aluminum, 4% to 14% molybdenum, manganese up to 2%, silicon up to 1%, carbon up to .20%, up to about 0.10% of the group consisting of titanium and zirconium, up to about 0.50% columbium, boron up to 0.010%, and remainder substantially all iron, with additional manganese, in excess of said 2%, substituted for said nickel on the basis of 2 for 1, the actual nickel content amounting to at least 3.5%, and the manganese content up to 19%, additional silicon substituted for chromium on the basis of l for 1 up to a total silicon content of 3%, tungsten substituted for molybdenum on a 1 for 1 basis up to a total tungsten content of 6%, and vanadium substituted for molybdenum and tungsten on a 1 for 1 basis up to a total vanadium content of 2%.

12. Chromium-nickel-aluminum cold-worked plate, sheet, strip, bars, rods, wire, shapes and tubes substantially free of delta-ferrite and hardenable by single heattreatment and consisting essentially of about: 4% to 14.5% chromium, 6% to 12% nickel, .5% to 2% aluminum, 4% to 14% molybdenum, carbon not exceeding .20%, and remainder substantially all iron, in which the extent of cold-reduction amounts to at least 25% reduction in area.

13. Chrornium-nickel-aluminum cold-rolled steel sheet and strip which is substantially free of delta-ferrite and hardenable by single heat-treatment, said sheet and strip consisting essentially of about: 4% to 14.5 chromium, 6% to 12% nickel, .5% to 2% aluminum, 4% to 14% molybdenum, carbon not exceeding .20%, and remainder substantially all iron, in which the extent of cold-rolling amounts to at least about reduction in area.

14. Chromium-nickel-aluminum cold-rolled steel sheet and strip which is substantially free of delta-ferrite and hardenable by single heat-treatment, said sheet and strip consisting essentially of about: 11% chromium, 8% nickel, 6% molybdenum, 1% aluminum, and remainder substantially all iron, in which the extent of cold-rolling amounts to at least about 60% reduction in area.

References (Iited in the file of this patent UNITED STATES PATENTS 2,505,763 Goller May 2, 1950 2,506,558 Goller May 2, 1950 2,590,835 Kirby et al. Apr. 1, 1952 2,958,617 Perry Nov. 1, 1960 2,999,039 Lula et al. Sept. 5, 1961 

1. A CHROMIUM-NICKEL-ALUMINUM STEEL WHICH IS SUBSTANTIALLY FULLY AUSTENITRIC AT ANNEALING TEMPERATURES AND WHICH IS HARDENABLE BY HEAT-TREATMENT, SAID STEEL CONSISTING ESSENTIALLY OF ABOUT: 4% TO 14.5% CHROMIUM, 6% TO 12% NICKEL, .5% TO 2% ALUMINUM, 4% TO 14% MOLYBDENUM, MANGANESE UP TO 2%, SILICON UP TO 1%, CARBON UP TO .20%, UP TO ABOUT 0.10% OF THE GROUP CONSISTING OF TITANIUM AND ZIRCONIUM, UP TO ABOUT 0.50% COLUMBIUM, BORON UP TO 0.010%, AND REMAINDER SUBSTANTIALLY ALL IRON, WITH ADDITIONAL MANGANESE, IN EXCESS OF SAID 2%, PARTIALLY SUBSTITUTED FOR SAID NICKEL ON THE BASIS OF 2 FOR 1, THE ACTUAL NICKEL CONTENT AMOUNTING TO AT LEAST 3.5%, AND THE TOTAL MANGANESE CONTENT UP TO 19%, ADDITIONAL SILICON SUBSTITUTED FOR CHRONIUM ON THE BASIS OF 1 FOR 1 UP TO A TOTAL SILICON CONTENT OF 3%, TUNGSTEN SUBSTITUTED FOR MOLYBDENUM O NA 1 FOR 1 BASIS UP TO A TOTAL TUNGSTEN CONTENT OF 6%, AND VANADIUM SUBSTITUTED FOR MOLYBDENUM AND TUNGSTEN ON A 1 FOR 1 BASIS UP TO A TOTAL OF VANADIUM CONTENT OF 2%. 